1. Field of the Invention
The present invention is broadly concerned with modified soy protein adhesives, and methods of production and use thereof. More particularly, the invention relates to such adhesives produced by the reaction of soy protein with a modifier selected from two classes of compounds in an aqueous system wherein the adhesives exhibit higher water resistance than non-modified soy protein adhesives. In the use of the adhesives of the invention, the bonds between the adherends have high shear strengths. The adhesives are preferably formulated by mixing a quantity of soy protein with an aqueous dispersion comprising the modifier and reacting the mixture; the resultant adhesive may be used in a liquefied form or dried to a reconstitutable powder.
2. Description of the Prior Art
Large quantities of adhesives are used annually for such things as interior and exterior applications of plywood and particle board, paper manufacturing, book binding, textile sizing, abrasives, gummed tape, and matches. Petroleum-based and soy protein-based adhesives are two general types of adhesives used in such applications. Soy protein adhesives known in the art generally lack the gluing strength and water resistance compared to petroleum-based adhesives. Most of the petroleum-based adhesives, however, contain phenol formaldehyde which is harmful to the environment. Also, petroleum resources are naturally limited and politically controlled. Furthermore, petroleum-based adhesives are not biodegradable thus resulting in an unwanted accumulation of waste. As a result, soy protein polymers are being reconsidered as an alternative adhesive to reduce the usage of petroleum polymers and to prevent environmental pollution.
In soy protein-based adhesives, the protein molecules are dispersed, and thus partially unfolded, in dispersion. The unfolded molecules increase the contact area and adhesion of the protein molecules onto other surfaces and entangle each other during the curing process to provide bonding strength. However, while soy protein-based adhesives are environmental friendly and are derived from soybeans which are renewable and more abundant than petroleum resources, the currently available soy protein-based adhesives lack the increased gluing strength and water resistance which are advantageous in typical adhesive applications.
The present invention overcomes the problems noted above, and provides a soy protein-based adhesive having increased bonding abilities and high water resistance. The adhesives of the invention are formed by modifying soy protein (SP), preferably derived from soy protein isolate (SPI) with certain compounds (modifiers).
Two classes of mild chemical modifiers are preferred in the preparation of the adhesives of the invention. One preferred modifier class includes saturated and unsaturated alkali metal C8-C22 sulfate and sulfonate salts. Even more preferred are saturated and unsaturated alkali metal C10-C18 sulfate and sulfonate salts. Saturated alkali metal C8-C22 sulfate and sulfonate salts include all alkali metal alkyl (such as octyl and dodecyl) C8-C22 sulfate and sulfonate salts. Unsaturated alkali metal C8-C22 sulfate and sulfonate salts include all alkali metal alkenyl (such as decenyl and octadecenyl) C8-C22 sulfate and sulfonate salts and all alkali metal alkynyl (such as octynyl and tetradecynyl) C8-C22 sulfate and sulfonate salts. Two particularly preferred modifiers in this class are sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS).
Another preferred class of modifiers for forming the adhesives of the invention are those compounds having the formula I: 
wherein each R is individually selected from the group consisting of H and C1-C4 saturated and unsaturated groups, and X is selected from the group consisting of O, NH, and S. The C1-C4 saturated and unsaturated groups refers to alkyl groups (both straight and branched chain) and unsaturated refers to alkenyl and alkynyl groups (both straight and branched chain). The preferred modifiers in this class are urea and guanidine complexed with hydrochloride.
Advantageously, the adhesives of the invention can be formed by utilizing mixtures of the foregoing modifiers. These mixtures can be within a class or they can contain modifiers from each class. For example, the modifier could be a mixture of urea and guanidine hydrochloride (which fall within the same class) or a mixture of urea and SDS.
When urea or guanidine hydrochloride (GH) is the modifier used to form the adhesives of the invention (hereinafter referred to as urea-modified soy protein adhesives or U-SPI and guanidine hydrochloride-modified soy protein adhesives or GH-SPI, respectively), the adhesives are preferably formed by preparing a mixture comprising (and preferably consisting essentially of) urea or guanidine hydrochloride (GH) and water with a quantity of soy protein thus forming a slurry or dispersion. Any source of soy protein (such as soybean concentrate or soybean meal) is suitable for making the adhesives of the invention, but a particularly preferred source of soy protein is soy protein isolate. The soy protein is preferably essentially free of urease, having less than about 10 activity units of urease. When urea is the modifier, the amount thereof should result in a urea content of at least about 6% by weight, preferably at least about 10% by weight, and more preferably at least about 15-18% by weight, based on the weight of the protein slurry. The amount of urea utilized is important in order to be certain that the protein molecules properly unravel in dispersion, thus increasing the contact area of the molecules as well as their adhesion onto other surfaces. These unraveled molecules will then entangle with each other during the curing process contributing to the increased bonding strength of the adhesives.
The resulting soy protein-urea or soy protein-GH aqueous dispersion is normally mixed for about 60 minutes. The forming of and mixing of the slurry is carried out at a temperature of from about 10 to about 80xc2x0 C., and preferably from about 20 to about 50xc2x0 C. Even more preferably, the forming and mixing of the dispersion takes place under ambient temperature and pressure conditions. The dispersion should have a pH of less than about 8, and more preferably less than about 7 (e.g., from about 2-7). After mixing, the reacted dispersion can be immediately used as an adhesive, or it can be freeze-dried, milled into a powder, and stored for later use. Or, if freeze-drying is not practical, the dispersion can be subjected to any known mechanism or process (such as spray drying) by which the moisture is substantially removed. This is just one example of how to form a modified soy protein adhesive utilizing a modifier (i.e., urea or GH) within this class of preferred modifiers. Any other modifiers having the formula I can be made in the same fashion, with appropriate process modifications depending upon the specific modifier chosen.
When sodium dodecyl sulfate or sodium dodecylbenzene sulfonate is the modifier used to form the adhesives of the invention (hereinafter referred to as SDS-SPI and SDBS-SPI adhesives, respectively), the process of forming the adhesives comprises mixing soy protein and water at room temperature for about 30 minutes to form a suspension or dispersion. Any source of soy protein (such as soybean concentrate or soybean meal) is suitable for making the adhesives, but a particularly preferred source of soy protein is soy protein isolate. SDS or SDBS is added to the suspension with the resulting dispersion being stirred for about 6 hours. Preferably, the SDS or SDBS is present in the resulting dispersion at a level from about 0.1 to about 15% by weight, and more preferably from about 0.5 to about 6% by weight based on the weight of said protein slurry. Preferably, the forming of the adhesive takes place under ambient temperature and pressure conditions. The final slurry should have the same pH ranges as the urea adhesives, and likewise such adhesives can be used in liquid or dried particulate form. This is just one example of how to form a modified soy protein adhesive utilizing a modifier (i.e., SDS or SDBS) which falls with the class of saturated and unsaturated alkali metal C8-C22 sulfate and sulfonate salts. Other adhesives made using modifiers within this class can be formed in a similar manner with appropriate process modifications depending upon the specific modifier chosen.
Regardless of the modifier utilized, the final dried adhesive powders should be small enough so that 90% of the powder particles pass through a 50 mesh screen, and preferably the powder particle size is such that 90% of the powder particles pass through a mesh screen having a size of at least about 100 mesh or higher (xe2x80x9chigherxe2x80x9d meaning e.g., 150 mesh or 200 mesh). The adhesive powders of the invention can be stored in powder form until the point-of-use. When the adhesive is needed, the powder is added to water (preferably distilled water) at a ratio of from about 1:20 to about 1:4, and preferably at a ratio of about 1:6 of SP powder:water. The powder is dispersed in the water at room temperature for about five minutes, preferably while heating to at least about 50xc2x0 C. The resulting aqueous adhesive can then be applied to an adherend by any conventional application means, such as by brushing the adhesive onto a surface of the adherend, or by placing the adhesive between and in contact with a pair of adherends. Preferably the adhesive application is followed by compressing of the adherends in order to facilitate adhesion.
In some applications, it may be desirable to modify a soy flour having at least about 50% protein to form the adhesives of the invention. Suitable modifiers for include all of those set forth above, with the exception of urea (because soy flours contains appreciable quantities of urease). Guanidine hydrochloride, SDS, and SDBS are particularly preferred for modifying soy flours. The modified soy flours have improved water resistance and gluing strengths as compared to non-modified soy flours.
The adhesives of the invention are particularly useful for adhering cellulosic components, such as those formed of wood or paper. The amount of adhesive necessary can be varied as required by the characteristics of the adherends. When the adhesives of the invention are applied to wooden adherends, the gluing strength (i.e., the shear strength) of the adhesive is at least about 30 kg/cm2, preferably at least about 50 kg/cm2, and more preferably about 65 kg/cm2.